Veterinary clinics are typically small businesses without access to air scavenging systems of the type found in large clinics or hospitals. Surgical operations and pre-surgical preparations routinely require the use of anesthetic substances in gaseous or vaporized form. Human exposure to commonly used anesthetic substances such as halothane, methoxyflurane or isoflurane has been linked to a myriad of health-related problems. Unless great care is exercised, undesirable levels of exposure to such anesthetic substances is possible, particularly in small clinics, because many of them use portable anesthetic delivery carts which are not provided with devices to capture waste gases.
In this context, the term "waste gases" should be understood to comprehend potentially harmful gaseous substances such as those listed above, whether these are released with the exhalations from the animal patient or due to venting from anesthetic supply containers.
In relatively large facilities there is often provision for adequate ventilation, and scavenging means are disposed to ensure that potentially harmful substances are promptly removed from the breathing zones immediately adjacent surgeons and their staff. This may be accomplished by providing flexible ducting to apply local suction, the purged air being released at a distance, e.g., outside the building where anesthetics are used in surgical activity. Such systems necessarily include fans driven by electric motors, i.e., they need power supplies and means for positively applying a suction to induce a flow to remove contaminated air. Such systems are therefore relatively expensive to purchase and install and require sophisticated maintenance to ensure efficient performance.
Reported estimates show that over 50,000 U.S. veterinarians, their technicians and assistants are routinely exposed to waste anesthetic agents. Chronic exposures to these agents have been linked to liver and kidney diseases, CNS effects, spontaneous abortions in females, congenital abnormalities and even cancer. See, for example, NIOSH: Criteria for a Recommended Standard--Occupational Exposure to Waste Anesthetic Gases and Vapors. DHEW(NIOSH) Pub. No. 77-140(1977).
In 1977, NIOSH published its criteria for limits on exposure to waste anesthetic agents, limiting exposure to N.sub.2 O at less than 25 ppm and halogenated agents at 2 ppm during administration of anesthetics to patients. If a halogenated agent is used in conjunction with N.sub.2 O, then the recommended exposure limit is 0.5 ppm for the halogenated agent. The American Conference of Governmental Industrial Hygienists (ACGIH) has recently adopted 50 ppm as its 8-hour Threshold Limit Value/Time-Weighted Average (TLV/TWA) for exposures to halothane and 75 ppm (TLV/TWA) to enflurane, two commonly used anesthetic agents. See, for example, American Conference of Governmental Industrial Hygienists: Threshold Limit Values and Biological Exposure Indices for 1988-1989. Cincinnati, Ohio (1988). The TLV for halothane is based on medical, environmental and epidemiological data collected for occupational exposure to trichloroethylene. The TLV for enflurane is based on the assumption that it is a safer anesthetic agent than halothane, and that no adverse effects are known at subanesthetic concentrations. See, for example, American Conference of Governmental Industrial Hygienists: Documentation of Threshold Limit Values and Biological Exposure Indices, 5th ed. (1986). No ACGIH threshold limit value currently exists for exposures to methoxyflurane. Also, at present, no OSHA permissible exposure level (PEL) exists for exposure to anesthetic agents; nor do specific recommendations exist for scavenging systems suitable for use in veterinary clinics.
An experimental study was conducted in which waste anesthetic gas exposures were determined, using a modified Miran 1A infrared spectrometer at five veterinary clinics operating within a Morgantown, W.V., facility. This device was slightly modified for these tests to increase the intrument's response time. Time-Weighted average exposures for unscavenged systems using methoxyflurane and halothane ranged from 0.5 to 45.5 ppm and 0.2 to 105.4 ppm, respectively. The purpose of this study was to evaluate "typical" exposure situations and to thereby develop procedures for control methods and parameters deemed effective and practical in reducing exposure levels to acceptable limits. The present invention, in partial reliance on this and other comparable studies, is intended to address such needs.
Scavenging systems have generally been the most recommended and widely proven method of reducing anesthetic gas exposures. A typical scavenging system consists of a hose attached to the pop-off valve of a cart carrying oxygen and anesthetic containers, the hose venting to a return air system or to the outside environment through a hole in the wall of the structure of the clinic. Such systems can cost between $200.00 and $500.00 to purchase and install, which may be prohibitive to small veterinary clinics. The overall effectiveness of such systems in such environments has not yet been fully established. A major problem in using such known technology is that its effectiveness is usually a function of the length of the hose by which local suction is applied or venting accomplished.
If anesthesia is being administered in only one location, then hose scavenging probably would be the control method of choice. Experience shows, however, that anesthesia is often given and maintained at many locations other than just the operating room. In practice, anesthesia is normally given during prepping and scrubbing, which in most cases lasts longer than the actual surgery time. It is also not uncommon to see more than one anesthetic gas cart being used at the same time in areas other than the operating room, e.g., a patient examination room. In summary, a single-location hose scavenging system may not be practical and, usually, the layout of the clinic may not allow multiple systems to be installed at affordable cost.
There is, therefore, a need for a compact, inexpensive, efficient and easy-to-maintain system mountable to individual anesthetic gas carts for removing ambient anesthetic substances from locations where humans would otherwise suffer repeated and unacceptably high levels of exposure to such substances.